Field of the Invention
The present invention relates to an improved system for controlling the recirculation of exhaust gases through an internal combustion engine to reduce or eliminate nitrous oxide emissions.
In U.S. Pat. No. 3,739,797 issued June 19, 1973 to Roland B. Caldwell, a pressure operated valve is provided for controlling the recirculation of engine exhaust gas back in to the engine intake manifold. The EGR valve is operated by pressure furnished from the intake manifold through a pressure regulator which regulates the valve operating pressure according to the flow rate of gas through the engine, preferably as indicated by the vacuum pressure established at a carburetor venturi. The regulator generally functions as a pressure amplifier in that the EGR valve operating pressure communicated to the EGR valve from the intake manifold varies as an amplified function of the input pressure signal derived from the carburetor venturi.
It is desirable to recirculate controlled flows of exhaust gas into the engine intake to provide optimum mixtures of air, fuel and exhaust gas flowing into the engine during various conditions of operation of the engine for minimizing the nitrous oxide content of the engine emissions. While the prior art systems have been quite successful in minimizing objectionable emissions by recirculating exhaust gas into the engine induction systems, the proportions of exhaust gas and air-fuel mixtures have not always been optimized for various operating conditions of the engines.
Most EGR valves are disposed in a crossover passage between the engine exhaust gas system and the intake air induction system. The pressure differential between the exhaust gas and the induction air varies according to the operating conditions of the engine and for this reason, with a given opening of the EGR valve, the flow of recirculated exhaust gas through the crossover passage varies according to the pressure differential between the intake and exhaust gas. This is particularly true in automotive vehicle engines which are operated at varying speeds and loads.
For example, when the engine is operating at a low cruising speed without a particularly great load, the throttle is positioned so that a relatively large magnitude vacuum pressure is present in the intake manifold while only a moderate regulator input signal pressure is provided. Under such conditions a moderate EGR valve operating pressure is produced by the regulator and the EGR valve positioned between its fully opened and fully closed positions to produced a theoretically moderate flow of recirculating exhaust gas as indicated by the level of the input pressure signal. However, because of the relatively large pressure differential between the exhaust and intake manifolds, the flow of exhaust gas through the EGR valve may be in excess of that required to produce an optimum mixture of air-fuel charge and recirculated exhaust gas.
When an engine is operated under a heavy load at relatively low speed, the regulator input pressure signal level may not be significantly greater than the input pressure signal level developed when the engine is operated at cruising speed under normal load; however, a substantial flow of recirculated exhaust gas through the EGR valve is normally required. Under this condition of engine operation the intake manifold pressure is of relatively low magnitude relative to atmospheric pressure and the pressure differential between the exhaust gas and the intake manifold may be sufficiently small that an inadequate flow of exhaust gas is recirculated when the EGR valve is in the open position dictated solely by the regulator input pressure signal. In such circumstances, in order to optimize the proportions of the air-fuel mixture and the recirculated exhaust gas flow, the EGR valve would have to be opened wider than its theoretical opening dictated by the input fluid signal.
As the engine speed approaches a wide open throttle condition when operating at high speed and/or under substantial load, the recirculation of exhaust gas should properly be terminated in accordance with predetermined intake manifold pressure characteristics since under substantially wide open throttle conditions, nitrous oxide is not produced as a combustion product and recirculation of exhaust gas is not necessary.